PROJECT SUMMARY Succesful implantation requires a receptive uterine endometrium. Any flaws in uterine endometrial receptivity can lead to implantation defects or failure. Currently, we have no clear answers for how the uterine endometrium achieves its receptivity, or how endometrial receptivity can be predicted or controlled. In this regard, the discovery of endometrial receptivity and non-receptivity markers offers a great promise. Implantation in primiparous or multiparous rodents only occurs in between two nodular (inter-nodular) areas, but not at nodular areas created at placental ejection sites after the end of pregnancy. This uterine region- based implantation strategy in parous mice could be due to loss of endometrial receptivity at the nodular, but not at the inter-nodular endometrium. These two dissimilar functional regions of the same uterine horn of parous mice offer an entirely new approach to discover morphological criteria and/or molecules pivotal to induce uterine nonreceptivity and/or receptivity. Thus, the goal of this application is to 1) demonstrate whether uterine receptive and non-receptive states can be discerned by morphological criteria of uterine luminal epithelial cells and/or vascular architecture, and 2) identify the molecule(s) whose upregulation or downregulation and/or no expression makes the uterine nodular sites, but not the inter-nodular sites, unsuitable for blastocyst implantation. Our working hypothesis is that morphological criteria and/or genes that are unique to either nodular or inter-nodular areas on day 4 of pregnancy could serve as excellent indicators for either poor or optimal uterine receptivity, respectively. Two Specific Aims will be pursued in primiparous (Para 1) mice on day 4 (the day of uterine receptivity) of their 2nd pregnancy to fulfill our goal. Specific Aim 1 will assess luminal epithelial surface morphology and blood vessels distribution changes at the nodular and inter-nodular uterine areas. Specific Aim 2 will discover molecule(s) responsible for uterine receptivity and/or non-receptivity by comparing genomic profiles of uterine tissues or cells from the nodular and inter-nodular areas over a period of time when uterine transition from the non-receptive state to the receptive state occurs. We will use multiple experimental approaches including scanning electron microscopy, laser captured microscopy, corrossion casting, punch biopsy, RNA- Sequencing, quantitative PCR, in situ hybridization, and immunohistochemistry to acomplish our goals. The outcome of our studies will provide useful information for the development of diagnostic and therapeutic tools that can be used for detection, prevention and treatment of female reproductive disorders, and improve technologies for assisted reproduction and contraception.